Just wanted to get some feedback on something that I've seen done and have heard various arguements on both sides of the coin. So if a game starts out and the GM wants to use same chapters such as maybe a whole squad as Black Templars then what's wong with making the game feel like a realistic kill team with same type chapters? I've heard that it takes away from the originality of Deathwatch with the player being able to decide from what Chapter he wants but for some reason it seems that it take away from a regimented feel to the game.

Id suggest if you are going for a chapter specific game then drop the deathwatch options - character creation seperates out what a character gets so you can just not give them the Deatchwatch training etc. Then simply run the game from the point of view of a single squad of Chapter-X, varying from small scale operations (chapters often send a single squad to deal with a distress signal or call for aid from a planetary governor) up to multiple-company battles (cinematically handled of course, otherwise there'd be far too much dice rolling!)

I agree with Kasatka, with one adendum. The grey knight rules in the deamon hunter book in dark heresy has rules for using them with deathwatch rules. In it is specifies that due to them all having the same training and not having the problem of conbining multiple chapters trainings into a single cohesive group, grey knights ignore cohesion. As grey knights replace the squad mode abilities with some psychic coolness of doom, it is fine. With the other chapters having squad mode abilities to use and such I reccomend the following option for a single chapter game: reduce the cohesion cost of all atack patterns and defensive stances to 0. This way they can still use thier abilities, free due to being in the same chapter and not having to coordinate as much.

Or if not free, have the base amount of cohesion points worked out as 10+ whatever it normally is. Then reduce the cost of all powers by 1. This means there will be a wide range of 0 cost powers for the players to use infinitely, and they'll have a large pool of cohesion to use the rest of the time.

I think it's important to keep cohesion mechanics though, as the unit can still be broken by a powerful/numerous/scary enough foe, regardless of chapter.

The core Deathwatch rules don't note any difference in cohesion or cohesion costs for a single chapter Killteam, despite that being entirely possible with the core rules. Food for thought.